fix: align commands, batching, reactions, and LRU with forcePerChat

Address review findings from self-review and codex:

- Commands (/reset, /cancel) now receive forcePerChat from the Discord
  adapter and resolve the correct per-thread conversation key
- Group batcher propagates forcePerChat to synthetic batch messages so
  debounced thread messages don't fall back to shared routing
- Reaction handler sets forcePerChat for thread-only reactions
- Session LRU eviction fires regardless of conversationMode so
  forcePerChat sessions don't accumulate without bounds
- Docs now correctly state thread-only overrides shared/per-channel
  modes (not disabled mode)

Written by Cameron ◯ Letta Code

"The best way to predict the future is to implement it." -- David Heinemeier Hansson

docs/configuration.md

@@ -461,7 +461,7 @@ channels:
 
 Thread messages inherit parent channel config, so child threads under `EZRA_CHANNEL_ID` use the same group rules.
 
-When `threadMode: thread-only` is set, each thread automatically gets its own isolated conversation (message history), regardless of the global `conversations.mode` setting. This prevents messages from different threads from being interleaved in the same conversation. Agent memory (blocks) is still shared across all threads.
+When `threadMode: thread-only` is set, each thread automatically gets its own isolated conversation (message history). This overrides `shared` and `per-channel` conversation modes so that messages from different threads are never interleaved. Agent memory (blocks) is still shared across all threads. (In `disabled` mode, all messages use the agent's built-in default conversation and thread isolation does not apply.)
 
 ### Finding Group IDs